Construction and initial characterization of Escherichia coli strains withfew or no intact chromosomal rRNA operons

The Escherichia coli genome carries seven rRNA (rrn) operons, each containi ng three rRNA genes. The presence of multiple operons has been an obstacle to many studies of rRNA because the effect of mutations in one operon is di luted by the six remaining wild-type copies. To create a tool useful for ma nipulating rRNA, we sequentially inactivated from one to all seven of these operons with deletions spanning the 16S and 23S rRNA genes. In the final s train, carrying no intact rRNA operon on the chromosome, rRNA molecules wer e expressed from a multicopy plasmid containing a single rRNA operon (prm). Characterization of these rrn deletion strains revealed that deletion of t wo operons was required to observe a reduction in the growth rate and rRNA/ protein ratio. When the number of deletions was extended from three to six, the decrease in the growth rate was slightly more than the decrease in the rRNA/protein ratio, suggesting that ribosome efficiency was reduced. This reduction was most pronounced in the Delta 7 prrn strain, in which the grow th rate, unlike the rRNA/protein ratio, was not completely restored to wild -type levels by a cloned rRNA operon. The decreases in growth rate and rRNA /protein ratio were surprisingly moderate in the rrn deletion strains; the presence of even a single operon on the chromosome was able to produce as m uch as 56% of wild-type levels of rRNA. We discuss possible applications of these strains in rRNA studies.